Presently, there are several types of semiconductor memory devices used for storing electronic data. They include Electronically Erasable Programmable Read Only Memory (EEPROM), Static Random Access Memory (SRAM) and Dynamic Random Access Memory (DRAM). Each memory type is composed of a plurality of memory cells, wherein each type of cell is uniquely designed to store electronic data (e.g. binary data) for retrieval at some later time.
Since each type of cell (i.e. DRAM cells, SRAM cells and EEPROM cells) has a different physical design from the other, one memory type may be more desirable for certain application than others. For example, an EEPROM is composed of a plurality of cells (EEPROM cells) which are relatively small in size. As a result, less silicon area is needed to manufacture an EEPROM memory than is required to manufacture SRAM having the same memory storage capacity. Thus, EEPROM memory can be less costly than SRAM having the same storage capacity. EEPROM memory can also have more storage capacity than SRAM due to its smaller size. Thus, an EEPROM is useful to provide electronic data storage in application wherein space and cost are at a premium. Such applications include watches, pagers and cellular phones.
Storing data in an EEPROM cell, however, is substantially slower than most other types of memory. That is, it takes more time to store data in an EEPROM memory cell than it takes for SRAM and DRAM memory cells. As a result, an EEPROM is not desirable for most memory applications where stored data must be quickly and frequently changed. Such applications include the operation of word processing software on a personal computer.
The types of memory that provide a faster write time than EEPROM include SRAM and DRAM devices. As described above, the increased speed of these devices is a function of their cell architecture. For example, a DRAM cell has a special capacitor for storing a charge, wherein the capacitor is used to control the data state of the DRAM cell. As a result, the capacitor is instrumental in providing a quick means of writing data within the cell. Also, when reading the content of a DRAM cell, the charge stored on the special capacitor must be large enough to be detectable by the device utilizing the memory. With the inclusion of such a capacitor, however, DRAM memories require a complicated and costly manufacturing process, and thus are not desirable for all high speed applications.
SRAM devices, although fast, are also not desirable for all high speed applications because an SRAM is composed of cells (SRAM cells) that are larger than both EEPROM cells and DRAM cells. Thus, for those applications where space or cost is at a premium, SRAM memories are less desirable than both EEPROM and DRAM devices.
Moreover, neither EEPROM, DRAM, nor SRAM are well suited for some of the many embedded applications for which electronic memory storage is required today. For example, for those applications requiring embedded memory in microcontrollers or logic chips, both speed and space are at a premium. As a result, many embedded applications require a memory that is fast, compact and inexpensive to make. As described above, neither DRAM, SRAM nor EEPROM devices provide both space and speed features for all such applications. As a result, there is a need for a memory device that is smaller than SRAM, less costly to produce than DRAM and faster to write to than EEPROM.